1. Field of the Invention
The present invention relates to a power output apparatus and a method of controlling the same. More specifically the present invention pertains to a power output apparatus for outputting power to a drive shaft and a method of controlling such a power output apparatus.
2. Description of the Related Art
In known power output apparatuses mounted on a vehicle, an output shaft of an engine is electromagnetically linked with a drive shaft, which a motor is attached to, by means of an electromagnetic coupling, so that power of the engine is output to the drive shaft (as disclosed in, for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814). In this power output apparatus, the vehicle starts driving by the function of the motor. When the revolving speed of the motor reaches a predetermined level, the power output apparatus supplies an exciting electric current to the electromagnetic coupling to crank the engine and carries out supply of a fuel into the engine and ignition with a spark, thereby starting the engine. After the engine starts, part of the power from the engine is output to the drive shaft via the electromagnetic coupling, in order to continue driving the vehicle. The residual power output from the engine is regenerated as electric power according to a slip of the electromagnetic coupling and may be stored into a battery as electric power used at the time of starting the vehicle or be used as electric power required for driving the motor. When the power output via the electromagnetic coupling is insufficient for the power to be output to the drive shaft, the motor is driven to supplement the shortage.
In the prior art power output apparatus, the electric power regenerated through the electromagnetic coupling or the electric power required for driving the motor is regulated by charging and discharging the battery. The efficiency of the whole power output apparatus accordingly depends upon the charge-discharge efficiency of the battery. When the power output from the engine is greater than the power required to the drive shaft, the electric power regenerated through the electromagnetic coupling becomes greater than the electric power consumed by the motor. In this case, the battery is charged with the surplus electric power. When the power output from the engine is smaller than the power required to the drive shaft, on the other hand, the electric power regenerated through the electromagnetic coupling becomes smaller than the electric power consumed by the motor. In this case, the battery is discharged to supplement the shortage of the electric power. When the power output from the engine is equal to the power required to the drive shaft, the electric power regenerated through the electromagnetic coupling becomes equal to the electric power consumed by the motor. In this case, the battery is neither charged nor discharged. The prior art power output apparatus, however, does not carry out such control.
In order to solve this problem, the applicant of the present invention has proposed power output apparatus for controlling operation of the engine to make the power output from the engine coincident with the power required to the drive shaft (for example, JAPANESE PATENT APPLICATION GAZETTE No. 7-225869).
Even under such control, the driving state of the engine, however, varies with the working temperature of the engine, the properties of the fuel supplied, the atmospheric temperature, and the atmospheric pressure. Such variation or an abnormality occurring in the engine may prevent the engine from outputting a desired power and cause the battery to be unexpectedly charged or discharged. This results in lowering the energy efficiency of the whole power output apparatus.